Conventional beam pumping installations for pumping fluid such as oil from underground locations utilize rods which are coupled in a continuous fashion to connect a surface pumping unit to an underground or subsurface downhole well pump for the purpose of transmitting mechanical energy from the surface equipment to the subsurface pump. The individual rods comprising the string are known as sucker rods and the plurality when coupled is referred to as a sucker rod string.
Subsurface oil well pumps are generally classified as either tubing or rod pumps. In the case of tubing pumps, the barrel is run on the tubing and the plunger is run on the rod string. In the case of rod pumps, the complete unit is run on the rod string. Rod pumps have the advantage of being more easily removed for servicing and are less susceptible to damage in running but they offer less working area for the plunger since the maximum bore of a rod pump is necessarily less than the maximum bore of a tubing pump for the same size tubing. In either case, however, pump travel length or plunger stroke is highly important in determining output, since the plunger stroke for any given pump when multiplied by the product of stroke rate and plunger area gives the volumetric productivity.
In the prior art publication "Well Design: Drilling and Production, Craft, B. C., Holden, W. R.,and Graves, E. P., Jr., Prentice-Hall Inc. 1962" it is taught that the effective plunger stroke downhole differs from the polished rod stroke; it is decreased by the effects of rod stretch resulting from fluid load and rod mass; and is increased by the effect of plunger overtravel. Since the magnitudes of these increases and decreases in stroke length are affected by the mechanical properties of the rods it is evident that the effective stroke downhole can be modified by suitable manipulation of the rod materials and characteristics, and this possibility has kead to considerable development effort in this area. In particular, it is interesting that modern data-logging and computational techniques, such as prescribed in SPE paper 588 by S. G. Gibbs presented at the Rocky Mountain Joint Regional Meeting, May 1963, of the Society of Petroleum Engineers of AIME permit the matching of sucker rod properties and the make-up of the sucker rod string to the operational parameters of a given well to achieve highly favorable pumping conditions, and hence, enhanced operational economics.
Early sucker rods were of all-metal construction as exemplified by U.S. Pat. No. 528,168 issued Oct. 30, 1894. Thereafter initial efforts to improve sucker rod performance were concerned with use of materials and design to resist corrosion and stress failure in view of the harsh environment of the well in which the rod is worked. These efforts are illustrated in prior art patents such as: U.S. Pat. No. 3,486,557 issued in 1969 to Harrison showing a rod comprising an inner cable surrounded by an encasement of molded plastic or fiberglass in an unspecified configuration wherein the end of the encasement has a conical recess to receive a splayed end of the cable which is held therein by metal introduced into the recess while molten and wherein the outer surface of the encasement is threaded to receive a connecting sleeve that serves to transfer load between adjacent sucker rods; U.S. Pat. No. 4,063,838 issued in 1977 to Michael showing a sucker rod having a solid steel core wrapped with resin-impregnated glass filaments in which the filaments form a stratified structure and the load transfer is via the outer surface of the wrapping in a manner similar to that described by Harrison. In this latter concept, however, the sheath material contains only helically wrapped filaments and is specifically designed to sustain compressive load in an attempt to maintain the core in a state of tension after the curing step.
It is interesting to note that as early as 1959 U.S. Pat. No. 2,874,937 to Higgins disclosed a sucker rod comprised of glass fibers held together by plastic resin. Intensive work has been undertaken in the field of fiberglass sucker rod design. Fiberglass is not seriously affected by corrosion, possesses a low specific gravity and has a high tensile strength-to weight ratio compared to steel.
In Paper SPE6851 presented at a technical meeting of SPE of AIME, Denver in October of 1977 Watkins and Haarsma described a continuous process for producing a high-volume-fraction glass rod in which glass filaments are collimated, saturated with resin, ordered into a circular configuration and cured. The paper presented data on the use of rods produced according to this process. The process has been referred to as the "pultrusion" process and the resulting rods have been referred to as "pultruded " fiberglass/resin composite rods.
Pultruded fiberglass sucker rods have a number of recognized positive attributes which include:
1. Higher Strength/Weight Ratio and Lower Rod Density than Steel Sucker Rods.
Lighter weight sucker rods allow the use of smaller pumpjacks and develop lower gear box loadings for a constant rate of production compared with those required for steel rods.
2. Good Corrosion Resistance/Low Electrical Conductivity.
Fiberglass/polyester composites have much greater resistance to corrosion than unprotected steel in the hostile environment found downhole. The downhole environment includes crude oil, H.sub.2 S, CO.sub.2 and water at temperature up to 200.degree. F., and furthermore, enhanced oil recovery techniques often result in increased concentration of corrosive elements. Rod strings consisting entirely of steel have been known to have useful lives of less than three months when employed in corrosive environment wells.
3. Opportunity for Increased Oil Well Productivity.
Fiberglass possesses an extensional modulus that is approximately 1/3 that of steel. While fiberglass is considered generally to be a stiff material, when fabricated into sucker rods and subsequently installed in a deep (approx. 3,000 to 8,000 ft.) well, the resulting structure is sufficiently compliant that the reciprocating motion of the rod string is affected to a considerable extent. That is, when the motion of the upper end of the rod string changes direction, the ratio of the inertial forces to the elastic forces is such that the lower end of the rod string tends to continue along the original direction. As a consequence the stroke of the lower end of the rod string can be considerably longer than the stroke at the upper end. This phenomenon, referred to as "overtravel," results in enhanced productivity for a given pump stroke and rate.
4. Relatively Simple to Fabricate.
Fiberglass can be pultruded along with a variety of resin systems (for example, polyester, vinyl ester or epoxy) on a continuous basis through a constant cross-section die. The pultruded rods are then cut to length and adhesively bonded to metal couplings.
While pultruded fiberglass sucker rods have the aforementioned attributes, they also possess some significant shortcomings. These include:
1. Coupling Bond.
Pultruded fiberglass sucker rods are bonded to the coupling at only one surface. This single interface between the composite rod body and the metal coupling is somewhat vulnerable and prone to premature failure.
2. Metal Couplings Exposed to Corrosive Environment.
Pultruded fiberglass rods are usually terminated with a steel coupling. This coupling is exposed to the sour environment of the oil well and is subject to corrosion and to the possibility of stress-corrosion failure.
3. Reduced Torsional Properties
The uniaxial character of the fiberglass in the pultruded rod does not provide strength in torsion. While sucker rods are not generally loaded in the torsional mode, torsional loads might be applied to unstick a downhole pump, and if the unsticking torque exceeds the torsional strength of the pultruded rod, it will fail in shear.
4. Poor Compressive Properties
Compression properties which are critical during sucker rod use include: local axial compression which occurs when the rod rubs against the tubing wall or if the downhole pump sticks; and compression impact if the rods part and the lower portion falls to the bottom of the well. Despite the inherent damping of the motion of this free falling section by the oil in the tubing, compression impact can cause temporary loading which is responsible for both fiber buckling and subsequent "brooming" of the fiberglass. Usually, a pultruded rod is rendered useless when this occurs.
Local compression can also occur when the operator sets the downhole pump to eliminate the condition known as gas pound. In this case, the pump is set to slightly tap the bottom and the local compression that results is small in magnitude, but is continual in nature, and it is reputed to cause premature failure over the long term.